Sagittarius debris in SDSS stripe 82

Zhu Ling ( 朱玲 ) & Martin. C. SmithCenter for Astrophysics, Tsinghua university

KIAA at Peking University

Motivation

• Sagittarius dwarf, being accreted by the Milky Way, forms lengthy tidal streams wrap entirely around the Milky way.

• It can provide important constrain to the shape, orientation, and mass of the Milky way dark matter halo.

• Velocity dispersion of the streams is an Important constraint.

• Relatively large sample of sag debris in a small region s

elected from SDSS stripe 82. • What constrains can this sample provide for the simulatio

n.

Majewski et al 2003 Niederste-Ostholt et al 2008

Star overdensity traces the sag streams

76

82

86

79

• Star overdensity traces the sag streams.

• The stream passes through S82, S86, but not S76, maybe S79

• Over 25,000 stars in stripe 82 have spectra, also about 10,000 in 79 and 86 in SDSS dr7.

• Overdensity of BHBs, BSs, K/M giants and MSTOs may trace sag streams (yanny et al 2000, yanny et al 2009).

Belokurov et al 2006

Select sample from S82

• Aim: A BHB+BS sample as clean as possible. • Selection Steps:

1) Color selection (A-color stars=BHB + BS + A type MS + BMP)

2) Cut in RA

3) Cut in g0 and surface gravity. (distance)

BHB: Blue Horizontal Branch starsBS: Blue StragglersBMP: Blue metal poor starsMS: main sequence stars

1) color selection

- 0.3 < g-r < 0 & 0.8 < u-g < 1.5 (yanny et al 2001)

g-r

A-color stars = BHBs + BSs + ***

u-g

2) cut in RA

• Lots of smooth halo stars exist. their velocity distribution centered at 0.

• Sag stream only passes

a small RA region. • Sag debris have a system

atic velocity offset.

RA

radi

al v

eloc

ity

3) Cut in distance• BHB: log(g) < 3.8, 17.2 < g < 18.7

typical absolute mag g0=0.7 (with dispersion ~ 0.2 mag)

• BS: log(g)> 3.8, 18.3 < g < 19.2

typical absolute mag g0 2.7 (with dispersion ~ 1 mag)

• faint: with no log(g) measurement,

g > 18.7. g=17.2 d=20kpc

g=18.7d=40kpc

Surface gravity log(g)

g=18.3d~ 10 kpc

appa

rant

mag

nitu

de

g

RA

rad

ial

velo

cit

y

15<RA<50: 416

288 have log(g)

Velocity distributionBHB: 76BS: 91faint: 123

• We can not kick out the smooth halo stars which exist at the same RA & distance.

•Another structure exists: a more compact, distant stream (Yanny et al. 2009, Newberg et al. 2009)

•To the sag stream, a velocity gradient exist along RA, not along distance.

• mean= -137 km/s. sigma=14.7 (19.3) km/s large error bar avg :13 km/s

BSs and faint stars have been shifted 2 mag left, so magnitude represents distance.

BHB+BS+fiant

Rad

ial

velo

city

Rad

ial v

eloc

ity

g

BHBs VS BSs

BS: 91sigma: 15 km/s (18.7)

BHB: 76sigma: 9.5 km/s (11.4)

BHB: 76BS: 91

surface temperature metallicity

BHB

BS

BS: colder, higher metallicityBHB: hotter, lower metallicity

BHBS VS BSs

Compare with previous resultManaco et al 2007 RGB Majewski et al 2004 M giants

8.3+-0.9 km/s 10.4+-1.3 km/s

BS ~ 15 km/s

BHB ~ 9.5 km/s

BHB+BS+faint ~ 14.7 km/s

How can the data constrain the simulation

Law et al 2010

How the velocity dispersion varies in the simulation

Fellhauer et al 2006

Rad

ial

velo

city

Vel

oci

ty

disp

ersi

on

Summary• We select BHBs + BSs sample of sag debris in SDSS dr7.

• BHBs and BSs show different velocity dispersion in this region.• BHBs: 76 stars. Velocity dispersion: 9.5 km/s• BSs : 91 stars. Velocity dispersion: 15 km/s• BHB+BS+faint: 76+91+123 stars. Velocity dispersion: 14.7 km/s.

• BSs are metal richer than BHBs, and BSs are with little lower temperature.

• The velocity dispersion of this sample are larger than previous results.

• Analysis of simulation data shows that the velocity dispersion varies along the stream.

• There are also a large number of red giants in the data, which we are looking into

• We are improving the analysis by using Markov Chain Monte Carlo techniques

Thank youThank you